Equalities campaigners have called for dramatic reform of party selection procedures after the proportion of women elected to the UK's devolved legislatures was one of the lowest on record.

Reform groups said the parties had to take positive action to increase the number of women in the Scottish parliament and the Welsh and Northern Irish assemblies, as last week's elections justified their fears there would be no improvement on equal rights.

Dr Ruth Fox, director of the Hansard Society's parliament and government programme, said: "Scotland and Wales have rightly been hailed as beacons of international progress on women's representation in the last decade. But these results are a blow to that record.

"The number of women elected to the Welsh assembly is going backwards, which is particularly disappointing when the assembly now has full law-making powers, and the progress at Holyrood is glacial, raising questions about what the commitment to 'new politics' really means."

In Thursday's devolved elections, there were 24 women elected in Wales, equalling the 40% achieved in 1999, which was the worst year for gender equality at the Cardiff Bay assembly. In 2003, the assembly became the first legislature in the world to have equal representation for both men and women.

In Scotland, the number of women elected was 45, resulting in the second lowest number of women MSPs in four Holyrood elections at 34.88%. The 2007 election produced the lowest number, at 33.33%.

In Northern Ireland, only 20, or 18.5%, of the Stormont assembly's 108 members were women – the worst figure for any of the UK's four legislatures. At the last general election, 22% of new MPs at Westminster were women – the highest figure ever achieved in the Commons.

These results are better than some of the most pessimistic predictions after the parties, including Labour, scrapped some of their most effective equal opportunities policies for internal political and electoral reasons, such as women-only shortlists.

An investigation by the Guardian in April found that less than 30% of all the candidates standing for major parties at Holyrood and Cardiff Bay would be women: the lowest figure on record for the Scottish parliament. In both places, 75% of candidates for constituency seats – the most favoured and prestigious seats – were men.

These findings confirmed the suspicions of the Institute for Welsh Affairs (IWA), the Hansard Society and Centre for Women and Democracy that the number of women elected last week would be the lowest on record. The IWA feared it could fall close to 30% in Wales.

The IWA argued last year that a minimum of 30% was needed to ensure a "critical mass" of women in any organisation. Kirsty Davies, the IWA's deputy director, said: "While this slip does not bring us perilously close to critical mass it is a worrying slide in the wrong direction."

Professor Laura McAllister, chair of IWA Women, an affiliated campaign group, said: "While this is disappointing, it is no great surprise as the main political parties have rowed back from positive action to promote women candidates.

"It is important that all organisations who value diversity, but especially within the political sphere, consider how best to ensure that the voices of women are properly heard."

Nan Sloan, director of the Centre for Women and Democracy, said: "This clearly isn't satisfactory, even allowing for Scotland's marginal increase. The political parties need to take a long hard look at what they're doing. They all signed up to improving women's representation but their lack of action means women are vulnerable at every election. As a consequence, it's easier to go backwards than forwards."

Thursday's election did lead to a slight improvement in the number of ethnic minority members of the Welsh assembly and Scottish parliament: in both places they doubled, to two. That brought Cardiff Bay's black representation close to the Welsh ethnic minority population of 3%. But Scotland's ethnic minority population stands at roughly 4%. Despite the election of Hanzala Malik for Labour on the Glasgow list, only 1.5% of Holyrood's 129 MSPs are non-white.

Malik's election and the election of several women MSPs was a direct consequence of the Scottish National party's dramatic landslide victory in constituency seats: this meant many white male Labour MSPs were not re-elected, giving women and Asian candidates put on the regional lists an unexpected success.

Ashok Viswanathan, from the equal representation campaign group Operation Black Vote, said: "It's lamentable that there are so few people to represent the diversity of Scotland in the 21st century. It is because of that we have consistently called for a comprehensive programme by all political parties, and by the institution itself, to recruit, promote and retain talent."

Scotland also had its first blind MSP elected, Dennis Robertson, the new SNP MSP for Aberdeenshire West.

• This article was amended on 10 May 2011. The original said that the number of women elected to Holyrood last week was 46, or 35.66% of the total. This has been corrected.